libnetfilter_cthelper: correct license GPLv2 -> GPLv2+

The main file of this package is src/libnetfilter_cthelper.c, which states
GPLv2+.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>